PURPOSE: To permit the flap of an envelope to be open positively by jetting air to the rear surface in the proximity of the flap of an envelope in an expansion area of the flap, and performing the expansion of the flap by means of a swayable absorbing pad.

CONSTITUTION: The flap expansion area P2 is provided with two absorbing pads 42 rotatably disposed integral with each other at distance left and right so as to be opposite to the flap E1 supplied to a predetermined position by a circular transportation mechanism 24. The absorbing pad 42 has a rotatable center at a folding center of the flap E1 and on the path of transportation. Between the absorbing pads 42 and a flap expansion ancillary arm 46, an air nozzle is arranged to jet air continuously to the rear surface of the envelope E to be delivered at an angle of elevation of the order of 65 degree toward the upstream side in the direction of transportation. As a result, an expansion of the flap E1 can be conducted easily and positively.